Update LPMS and use livepeer's custom FFmpeg fork

[jailuthra]

What does this pull request do? Explain your changes. (required)

See title

Specific updates (required)

See commits

How did you test each of these updates (required)

• Tested LPMS in staging with the ffmpeg version introduced in avcodec/nvenc: use framerate if available FFmpeg#2
• Manual + staging testing done for go-livepeer in a B-O-T setup prior to recent changes. More testing needed.

Does this pull request close any open issues?

Fixes livepeer/lpms#232

Checklist:

• Read the contribution guide
• make runs successfully
• All tests in ./test.sh pass
• README and other documentation updated
• Pending changelog updated

[yondonfu]

Pushed the following commits:

7792aeb to update the go mod files to point to the latest commit on LPMS master (with livepeer/lpms#229 merged)

cb944d1 to update install_ffmpeg.sh to point to the latest commit in livepeer/ffmpeg (with livepeer/FFmpeg#2 merged)

09b6477 to update CHANGELOG_PENDING with an entry for this PR (and the credit to @jailuthra for authoring everything)

[yondonfu]

Tested that passthrough FPS and non-passthrough FPS both work - LGTM

Resolved by pointing to commit hash in install_ffmpeg.sh